1. Field of the Invention
The present invention relates to an exhaust gas purifying device for an in-cylinder injection spark ignition type internal combustion engine, and more particularly, to techniques of improving the exhaust gas purifying efficiency to enhance the effect of reducing the emission quantity of harmful matter contained in exhaust gas.
2. Description of the Related Art
As one of techniques for reducing harmful matter (including not only unburned matter such as HC, CO, and H2 but smoke, NOx, etc.) contained in exhaust gas, there has been known an exhaust gas purifying technique utilizing reaction with the aid of catalyst.
This exhaust gas purifying technique is, however, disadvantageous in that unburned matter such as HC is discharged into the atmosphere before the catalyst is activated. The disadvantage can pose a grave problem because the quantity of harmful matter discharged before the catalyst is activated occasionally accounts for 90% of the total emission quantity during cold-mode operation.
To solve the problem, a two-stage combustion technique (or two-stage injection technique) for an in-cylinder injection spark ignition type internal combustion engine has been proposed. According to this technique, fuel injection is carried out in two stages, that is, a main injection (with the air/fuel ratio so set as to provide an ultra-lean mixture) and an auxiliary injection which is performed in or after the expansion stroke to cause oxygen remaining after a main combustion (stratified combustion) following the main injection to react with unburned fuel of the auxiliary injection in an exhaust system (including parts of the engine from the combustion chamber to the catalytic converter), thereby reducing harmful matter and quickening the activation of the catalyst.
There has also been proposed a technique of accelerating the reaction by increasing the exhaust pressure (by restraining the exhaust gas flow) and thereby quickening the activation of the catalyst in cold mode, as disclosed in Unexamined Japanese Patent Publications (KOKAI) No. 3-117611 and No. 4-183921, for example.
Further, a combination of these techniques also has been developed wherein the two-stage combustion is carried out while at the same time the exhaust pressure is increased, to quicken the activation of the catalyst in cold mode.
In the case where the two-stage combustion is performed, however, although the reaction in the exhaust system can be drastically accelerated and the catalyst can be activated quickly, a problem arises in that the auxiliary injection leads to a correspondingly lower fuel efficiency because the fuel injected in the auxiliary injection does not directly contribute to the production of engine torque.
Also, in the case of the two-stage combustion, the air/fuel ratio needs to be controlled such that the air/fuel mixture burned in the main combustion following the main injection should be ultra-lean. There is naturally a limit to the engine torque that can be produced, and if the exhaust pressure is increased, the exhaust resistance also increases, making it difficult even to maintain the limit. Such a limit on the engine torque is undesirable because the driver""s demand for acceleration may fail to be satisfied.
An object of the present invention is to provide an exhaust gas purifying device for an in-cylinder injection spark ignition type internal combustion engine, which device is capable of minimizing lowering of the fuel efficiency even during cold-mode operation, and improving the exhaust gas purifying efficiency to restrain the emission quantity of harmful matter while at the same time ensuring acceleration performance.
To achieve the object, the present invention provides an exhaust gas purifying device for an in-cylinder injection spark ignition type internal combustion engine including an injection valve for injecting fuel directly into a combustion chamber, fuel injection control means for controlling the injection of fuel by the injection valve, and air/fuel ratio control means for controlling burn air/fuel ratio. The exhaust gas purifying device comprises a catalytic converter arranged in an exhaust passage for purifying harmful matter in exhaust gas, catalyst activation determining means for determining an activated state of the catalytic converter, compression-stroke slight-lean operation means for causing the air/fuel ratio control means to control the burn air/fuel ratio to produce a slight lean air/fuel mixture equivalent to or slightly leaner than a stoichiometric air/fuel mixture and also causing the fuel injection control means to inject fuel in a compression stroke, to thereby carry out a compression-stroke slight-lean operation, and exhaust gas flow control means for restraining an exhaust gas flow in the exhaust passage, wherein when it is judged by the catalyst activation determining means that the catalytic converter is not in the activated state, the compression-stroke slight-lean operation is performed by the compression-stroke slight-lean operation means and also the exhaust gas flow in the exhaust passage is restrained by the exhaust gas flow control means.
It has been thought hitherto that in the case of an in-cylinder injection spark ignition type internal combustion engine, if fuel is injected in the compression stroke, the spark plug is likely to be wetted because fuel is injected in the vicinity of the spark plug. As a result of experiments conducted by the applicant hereof, however, it was found that when fuel was injected in the compression stroke especially while the engine was in a cold state, the injected fuel actually showed a satisfactory spray pattern with less wetting of the spark plug. It was also found that a large quantity of CO (carbon monoxide) was produced while at the same time ensuring the engine torque. In accordance with the findings, when the engine is in a cold state, the compression-stroke slight-lean operation is carried out wherein the burn air/fuel ratio is so controlled as to produce a slight lean air/fuel mixture equivalent to or slightly leaner than the stoichiometric air/fuel mixture and fuel is injected in the compression stroke, thereby to discharge CO into the exhaust passage as unburned matter together with the residual oxygen (O2), and also the exhaust gas flow is restrained at the same time.
Consequently, while the engine is in a cold state and the catalytic converter is not yet activated, the exhaust gas can be made to contain a large quantity of CO as unburned matter along with O2, without carrying out the auxiliary injection of the two-stage combustion. CO reacts with O2 more easily than HC, so that the reaction in the exhaust system is further accelerated also with the aid of the exhaust gas flow restraining effect. It is therefore possible to improve the exhaust gas purifying efficiency with the air/fuel mixture as a whole set to a slightly lean mixture to restrain the fuel efficiency from lowering while at the same time ensuring the engine torque, and also to quicken the activation of the catalytic converter.
The exhaust gas purifying device of the present invention may further comprise exhaust port ambient temperature detecting means for detecting temperature of or ambient temperature of an exhaust port extending from the combustion chamber to the exhaust passage. When it is judged by the catalyst activation determining means that the catalytic converter is not in the activated state and also the temperature or ambient temperature of the exhaust port detected by the exhaust port ambient temperature detecting means is higher than or equal to a predetermined temperature, the compression-stroke slight-lean operation is carried out and also the exhaust gas flow is restrained.
The experiments conducted by the applicant hereof confirmed that the reaction of CO in the exhaust system was accelerated inside the exhaust port into which the exhaust gas flows directly from the combustion chamber. The rate of the reaction increases with increase in the temperature of the exhaust port, and accordingly, when the engine is in a cold state and if the ambient temperature of the exhaust port is higher than or equal to the predetermined temperature, the compression-stroke slight-lean operation is carried out to discharge CO into the exhaust passage as unburned matter together with the residual O2, and also the exhaust gas flow is restrained.
Thus, while the engine is in a cold state and the catalytic converter is not yet activated, the reaction of CO with O2 in the exhaust system is more appropriately accelerated, whereby the exhaust gas purifying efficiency can be further improved while restraining the fuel efficiency from lowering and at the same time ensuring the engine torque, and also the catalytic converter can be activated more quickly.
The exhaust gas purifying device of the present invention may further comprise two-stage combustion operation means for supplying fuel for main combustion into the combustion chamber and then again supplying fuel into the combustion chamber at a time after extinction of flame produced by the main combustion and before opening of an exhaust valve, to thereby carry out a two-stage combustion operation. When it is judged by the catalyst activation determining means that the catalytic converter is not in the activated state, one of the two-stage combustion operation and the compression-stroke slight-lean operation is selectively carried out by the two-stage combustion operation means or the compression-stroke slight-lean operation means, and also the exhaust gas flow is restrained.
Accordingly, the two-stage combustion operation or the compression-stroke slight-lean operation can be suitably selected in accordance with the operating conditions of the engine. Where the two-stage combustion operation is selected and also the exhaust gas flow is restrained, the exhaust gas purifying efficiency can be improved while preferentially quickening the activation of the catalyst. On the other hand, where the compression-stroke slight-lean operation is selected and also the exhaust gas flow is restrained, the exhaust gas purifying efficiency can be improved while preferentially restraining the fuel efficiency from lowering and at the same time ensuring the engine torque.
Also, in the exhaust gas purifying device according to the present invention, when it is judged by the catalyst activation determining means that the catalytic converter is not in the activated state, the two-stage combustion operation may be carried out first, followed by the compression-stroke slight-lean operation.
Thus, when the catalytic converter is not in the activated state, the two-stage combustion operation is carried out first while at the same time the exhaust gas flow is restrained, whereby the catalyst can be quickly activated. At the time when the catalyst has been activated to a certain degree, the operation mode is switched to the compression-stroke slight-lean operation, whereby the exhaust gas purifying efficiency can be effectively improved while restraining the fuel efficiency from lowering and also securing the engine torque.
In the exhaust gas purifying device according to the present invention, when the compression-stroke slight-lean operation is carried out, the exhaust gas flow control means sets a restraint quantity for restraining the exhaust gas flow to a value greater than that applied when the two-stage combustion operation is carried out.
Specifically, during the two-stage combustion operation in which the air/fuel ratio for the main combustion is set so as to produce an ultra-lean mixture, the quantity of intake air is large and thus the flow rate of exhaust gas is high. Thus, even if the restraint quantity for restraining the exhaust gas flow is relatively small, the exhaust gas flow can be easily restrained to accelerate the reaction in the exhaust system. However, during the compression-stroke slight-lean operation, the flow rate of exhaust gas is low, and therefore, the restraint quantity for restraining the exhaust gas flow is increased to reliably restrain the exhaust gas flow.
Consequently, the exhaust gas purifying efficiency can be more effectively improved while restraining the fuel efficiency from lowering and at the same time ensuring the engine torque.
Also, the exhaust gas purifying device of the present invention may further comprise load detecting means for detecting a load on the engine. When the load of the engine detected by the load detecting means is higher than or equal to a predetermined value, the compression-stroke slight-lean operation is carried out preferentially over the two-stage combustion operation.
Thus, when acceleration is demanded even while the catalytic converter is still not in the activated state, the compression-stroke slight-lean operation is carried out preferentially over the two-stage combustion operation, whereby the exhaust gas purifying efficiency can be improved while restraining the fuel efficiency from lowering and at the same time ensuring the engine torque.
The exhaust gas purifying device of the present invention may further comprise intake-stroke slight-lean operation means for causing the air/fuel ratio control means to control the burn air/fuel ratio to produce a slight lean air/fuel mixture equivalent to or slightly leaner than a stoichiometric air/fuel mixture and also causing the fuel injection control means to inject fuel in an intake stroke, to thereby carry out an intake-stroke slight-lean operation. When it is judged by the catalyst activation determining means that the catalytic converter is not in the activated state, one of the compression-stroke slight-lean operation and the intake-stroke slight-lean operation is selectively carried out by the compression-stroke slight-lean operation means or the intake-stroke slight-lean operation means, and also the exhaust gas flow is restrained.
Accordingly, the compression-stroke slight-lean operation or the intake-stroke slight-lean operation can be suitably selected in accordance with the operating conditions of the engine. Where the compression-stroke slight-lean operation is selected and also the exhaust gas flow is restrained, the exhaust gas purifying efficiency can be improved while preferentially restraining the fuel efficiency from lowering and at the same time ensuring the engine torque. On the other hand, where the intake-stroke slight-lean operation is selected and also the exhaust gas flow is restrained, the exhaust gas purifying efficiency can be improved while preferentially securing the engine torque.
Also, in the exhaust gas purifying device according to the present invention, when it is judged by the catalyst activation determining means that the catalytic converter is not in the activated state, the compression-stroke slight-lean operation may be carried out first, followed by the intake-stroke slight-lean operation.
Thus, when the catalytic converter is not in the activated state, the compression-stroke slight-lean operation is carried out first while at the same time the exhaust gas flow is restrained, whereby sufficient engine torque can be secured while restraining the fuel efficiency from lowering. At the time when the catalyst has been activated to a certain degree, the operation mode is switched to the intake-stroke slight-lean operation, whereby the engine torque can be secured more reliably while at the same time the exhaust gas purifying efficiency can be effectively improved.
In the exhaust gas purifying device according to the present invention, when the intake-stroke slight-lean operation is carried out, the exhaust gas flow control means sets a restraint quantity for restraining the exhaust gas flow to a value greater than that applied when the compression-stroke slight-lean operation is carried out.
Specifically, during the compression-stroke slight-lean operation in which stratified combustion takes place, the quantity of intake air is relatively large and thus the flow rate of exhaust gas is relatively high. Thus, even if the restraint quantity for restraining the exhaust gas flow is somewhat small, the exhaust gas flow can be restrained to accelerate the reaction in the exhaust system. However, during the intake-stroke slight-lean operation, the exhaust gas flow rate is even lower, and therefore, the restraint quantity for restraining the exhaust gas flow is increased to reliably restrain the exhaust gas flow.
Consequently, the exhaust gas purifying efficiency can be more effectively improved while restraining the fuel efficiency from lowering and at the same time securing sufficient engine torque.
Also, the exhaust gas purifying device of the present invention may further comprise load detecting means for detecting a load on the engine. When the load of the engine detected by the load detecting means is higher than or equal to a predetermined value, the intake-stroke slight-lean operation is carried out preferentially over the compression-stroke slight-lean operation.
Thus, when acceleration is demanded even while the catalytic converter is still not in the activated state, the intake-stroke slight-lean operation is carried out preferentially over the compression-stroke slight-lean operation, whereby the exhaust gas purifying efficiency can be improved while restraining the fuel efficiency from lowering and at the same time securing sufficiently high engine torque.
In the exhaust gas purifying device according to the present invention, the catalyst activation determining means uses, as a parameter, at least one of time elapsed from start of the engine, temperature of cooling water of the engine, temperature of lubricating oil of the engine, temperature of the catalytic converter, flow rate of the exhaust gas, and total quantity of heat applied to the catalytic converter, to determine the activated state of the catalytic converter.
Thus, the activated state of the catalytic converter can be determined with ease based on the time elapsed from the start of the engine, the temperature of the cooling water of the engine, the temperature of the lubricating oil of the engine, the temperature of the catalytic converter, the flow rate of the exhaust gas, the total quantity of heat applied to the catalytic converter, etc.
Also, in the exhaust gas purifying device according to the present invention, the exhaust gas flow control means reduces either cross-sectional area of the exhaust passage or opening of an exhaust choke valve arranged in the exhaust passage, to thereby restrain the exhaust gas flow in the exhaust passage.
Accordingly, the exhaust gas flow in the exhaust passage can be easily restrained by reducing the cross-sectional area of the exhaust passage or the opening of the exhaust choke valve arranged in the exhaust passage.
In the exhaust gas purifying device according to the present invention, the load detecting means detects the load of the engine based on at least one of throttle valve opening, intake manifold pressure, and intake air quantity.
Thus, the load of the engine can be detected with ease based on the throttle valve opening, the intake manifold pressure, the intake air quantity, etc.